The present invention relates to an optical transmission module, and it relates in particular to a circuit system of a driving circuit for a laser diode in a transmitting circuit for an optical communication system.
The optical transmission module which has been known to the public is composed of a differential amplifier for waveform shaping of an input signal, a differential type current switch which generates a modulating current for driving a laser diode upon receipt of the output signal of the differential amplifier, a variable current supply which varies the current to be supplied to the differential type current switch and a bias current supply for supplying a DC bias current to the laser diode. Some of the above-mentioned variable current supplies can be adjustable in accordance with the setting of an optical output or luminous efficiency characteristics of a laser diode (refer to Japanese Patent Laid-open No. 5-218543).
A prior art optical transmission module is shown in FIG. 5.
FIG. 5 is a circuit diagram of the prior art optical transmission module. In the case of the optical transmission module shown in FIG. 5, a laser diode 200 is disposed in the right vicinity of a laser driving circuit 208. A differential type current switch 206 is composed of transistors TR1 and TR2 and it is driven by a differential signal voltage input through input terminals 204 and 205 of the differential type current switch 206. The current supplied by the modulating current supply 207 is divided and supplied to a resistor 203 and a laser diode 200 by the differential type current switch 206, and the laser diode 200 is modulation-driven according to an input signal.
Further, a bias current supply 202 supplies a bias current to the laser diode 200. For the purpose of preventing the leakage of a signal current, a bias tee 201 is inserted between the bias current supply 202 and the laser diode 200.
If high-speed modulation of the laser diode 200 is carried out by use of the present circuit system, it is absolutely necessary to dispose the laser diode 200 at a position sufficiently nearer to the laser driving circuit 208 in order to prevent the deterioration of the waveform of the signal current caused by the reflection thereof. It is also necessary to use the wiring of enough low inductance to prevent the voltage drop caused by the inductance of the wiring. The phenomena mentioned in the above, mean that the higher becomes the signal speed the more difficult becomes the technique of mounting of parts. When the present circuit is used for an optical transmission module for which high speed modulation is always required, a highly precise mounting technique is needed, which naturally raises the manufacturing cost of an optical transmission module.
Further heat generating components are mounted close up together, so that it is difficult to effectively dissipate the heat generated by the laser driving circuit 208 and the laser diode 200.
Next, an optical transmission module which can be mounted simpler than the above module will be explained referring to FIG. 6.
FIG. 6 shows a circuit diagram of another example of a conventional optical transmission module. In the case of the present optical transmission module, an impedance controlled high frequency transmission line is used for the wiring between a laser driving circuit 312 and a laser diode 300. A termination resistor 303 is inserted between the laser diode 300 and the transmission line 304 to prevent the reflection of a signal current at the end of the transmission line 304.
A bias current is supplied to the laser diode 300 through a low pass filter 301 from the bias current supply 302. A capacitor 305 is inserted in the transmission line 304 to prevent the voltage drop caused by the bias current which may influence on the operation of the differential type current switch 308.
In order to secure the bias voltage of the differential type current switch 308, a resistor 306 is inserted between the power supply and the transistor TR2 in the differential type current switch 308. Further, a resistor 307 is inserted between the power supply and the transistor TR1 to prevent the deterioration of the current waveform to be supplied to the laser diode 300.
Owing to the adoption of the present configuration, it is made possible to dispose the laser diode 300 and the laser diode driving circuit 312 apart from each other and also it is made possible to drive the laser diode at a high speed with a simple mounting technique and cheep mounting cost. That is, in the present configuration, there is no need to dispose the laser driving circuit 312 and the laser diode 300 at near by places from each other, and since it is possible to pack only the laser diode 300 into a package and make it a transmitting subassembly, so that it is made possible to actually mount necessary components at a cheep price and with an easy technique.
On the other hand, when the present circuit is adopted, a resistor 303 is inserted in series with the laser diode 300 to terminate the transmission line 304. The value of the resistor is in the range of 25 to 50 ohms caused by a limitation on the design of a transmission line, and it is a large resistance in comparison with that of the laser diode. When the laser diode is driven, the voltage drop at the output terminal of the differential type current switch 308 caused by the resistor becomes large. Since a certain bias voltage is needed to make the differential type current switch 308 and the current supply 311 operate normally. Therefore, in order to obtain an output having a good waveform, the laser diode has to be driven in such a manner that the voltage at the output terminal of the transistor TR2 of the differential type current switch 308 is always kept not less than a certain level. Consequently, when the laser diode is driven according to the present system, in order to compensate the voltage drop, a higher power supply voltage is needed in comparison with that of the system shown in FIG. 5.
Further, since the resistor 303 consumes about half of the current supplied by the differential type current switch 308, the modulation current supply 311 has to supply twice the current to be utilized by the laser diode 300. Therefore, the current consumed by the laser driving circuit 312 has to be doubled comparing to that of the system shown in FIG. 5.
The prior art related to bidirectional drive of laser diodes, etc. using differential signals are known from the following patent documents: Japanese Patent Laid-open Nos. 07-334859, 2001-015854, 2001-111167 and 2002-277840.
As mentioned in the above, when high-speed laser modulation is to be performed in the prior art shown in FIG. 5, the apparatus becomes complicated and high cost, and further since heat generating components are aggregated, it becomes difficult to dissipate the generated heat effectively.
In the case of the prior art shown in FIG. 6, the mounting of components can be done without a difficult technique; accordingly the manufacturing cost can be low. However, the voltage drop is increased caused by the resistor, which requires a higher power supply voltage in comparison with that of the system shown in FIG. 5. It means larger power consumption in the system.